Lithography is a patterning technique using material photosensitive to light such as laser beams. By lithography, thermal change of the material occurs over the whole area inside the processing laser beam spot, and a pattern is formed whose size is the same as the processing laser beam spot size. By heat-mode lithography, which is based on the manufacturing technique for optical discs, thermal change of the material occurs only at the center of the laser beam spot, and the pattern can be formed whose size is far smaller than the conventional pattern size (Figure 1). We focused on an inorganic compound material (ZnS-SiO2) that has been used as a protective layer for rewritable DVD, and by applying a characteristic of the inorganic material to a patterning technique that the thermal reaction super-sensitively occurs above a certain temperature, not only small in size but also steep and clear pattern edges can be formed (Figure 2). Fine patterns can be formed by a simple process involving laser irradiation and wet etching (Figure 3).
Figure 1: Feature of heat-mode lithography
Figure 2: Patterning material and pattern shape
Figure 3: Patterning method of ZnS-SiO2 thin film
The pattern size is controlled by changing laser irradiation conditions, and a pattern can be formed whose size is approximately one-fourth the size of the laser beam spot (Figure 4). In addition, quartz patterns are formed by etching the base quartz substrate while using the ZnS-SiO2 pattern as a mask which is achieved by heat-mode lithography described above. The quarts pattern shape can be controlled by changing etching conditions (Figure 5). Further, by using the patterned quartz substrate as a master mold for printing fine patterns, replication of the patterned substrate is available with reduced cost by nano-imprinting and other processes.
Figure 4: SEM (Scanning electron microscopy) image of ZnS-SiO2 pattern
Figure 5: SEM image of quartz patternFor all of the Figures 5(a)-5(d), pattern of Fig.4(c) whose diameter is 300nm is used as a mask and the mask size is the same.
Pattern density (pattern pitch) is different between Fig.5(a) and Fig.5(d), and Fig.5(b) and Fig.5(c).
In the case of conventional lithography technique, the pattern size is approximately the same as the processing laser beam spot size. Consequently, it is necessary to shorten the wavelength of the laser beam to reduce the pattern size. Therefore, expensive lithography equipment using an e-beam or an x-ray as a light source is needed in order to achieve a fine pattern whose size is less than 100nm, making the process high-cost. On the other hand, in the case of heat-mode lithography which is a patterning technique exceeding the resolution limit of the laser beam, fine pattern can be achieved with an inexpensive laser irradiation equipment (Figure 6). Furthermore, using a high-speed rotating stage on which the processing subject is placed within the laser irradiation setup, large-area and high-speed laser irradiation is possible.
Figure 6: Pattern size and process cost
A fine pattern that can be achieved by the heat-mode lithography suppresses reflection of visible light (Figure 7). In the case of functional devices having light emitting/receiving function (such as surface emitting light source and sensors), conventionally an anti-reflection function of dielectric multi-layer thin-film coating has been employed, which is expensive, in order to improve light emitting/receiving efficiency. However, new technology is required that brings in a similar effect at reduced cost. Heat-mode lithography, by which a fine pattern can be achieved with low cost as well as large-area patterning, is expected to be applied as a fine patterning process to improve light emitting/receiving efficiency of the functional devices.
Figure 7: Anti-reflection effect of patterned surface